Migrate skip checks to --EXTENSIONS--, p1

For rationale, see https://github.com/php/php-src/pull/6787

Extensions migrated in this part:
* bcmath
* bz2
* calendar
* c

Migrate skip checks to --EXTENSIONS--, p1

For rationale, see https://github.com/php/php-src/pull/6787

Extensions migrated in this part:
* bcmath
* bz2
* calendar
* com_dotnet
* ctype

Closes GH-6797.

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Promote some warnings in BCMath to Errors

Also do a bit of refactoring at the same time.

Closes GH-6105

make bcpowmod stricter by not returning false, instead throw exception

Closes GH-5747